Question

I’ve written a small tiling game engine with OpenGL and C, and I can’t seem to figure out what the problem is. My main loop looks like this:

void main_game_loop()
{
    (poll for events and respond to them)
    glClear(GL_COLOR_BUFFER_BIT);
    glPushMatrix();
    draw_block(WALL, 10, 10);
}

draw_block:

void draw_block(block b, int x, int y)
{
    (load b's texture from a hash and store it in GLuint tex)
    glPushMatrix();
    glTranslatef(x, y, 0);
    glBindTexture(GL_TEXTURE_2D, tex);

    glBegin(GL_QUADS);
    //BLOCK_DIM is 32, the width and height of the texture
    glTexCoord2i(0, 0); glVertex3f(0, 0, 0);
    glTexCoord2i(1, 0); glVertex3f(BLOCK_DIM, 0, 0);
    glTexCoord2i(1, 1); glVertex3f(BLOCK_DIM, BLOCK_DIM, 0);
    glTexCoord2i(0, 1); glVertex3f(0, BLOCK_DIM, 0);
    glEnd();
    glPopMatrix;
}

initialization function: (called before main_game_loop)

void init_gl()
{
    glViewport(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT);

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluOrtho2D(0, SCREEN_WIDTH, SCREEN_HEIGHT, 0);

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();

    glClearColor(0, 0, 0, 0);

    glEnable(GL_TEXTURE_2D);
    glEnable(GL_BLEND);
    glDisable(GL_DEPTH_TEST);
    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
}

When run, this displays a black screen. However, if I remove the glViewport call, it seemingly displays the texture, but huge and in the corner of the window. Screenshot:

The texture IS being drawn correctly, because if I scale out by a huge factor, I can see the entire image. The y-axis also seems to be flipped from what I used in the gluOrtho2D call (discovered by making events add or subtract from x/y coordinates of the image, subtracting from the y coordinate causes the image to move downward). I’m starting to get frustrated, because this is the simplest possible example I can think of. I’m using SDL, and am passing SDL_OPENGL to SDL_SetVideoMode. What is going on here?

Answer 1

Looks like a problem with glViewport, but just to be sure, did you try clearing the color buffer to purple?

I’ve always thought of glViewport as a video/windowing function, not actually part of OpenGL itself, because it is the intermediate between the window manager and the OpenGL subsystem, and it uses window coordinates. As such, you should probably look at it along with the other SDL video calls. I suggest updating the question with the full code, or at least with those parts relevant to the video/window subsystem.

Or is it that you omitted to call glViewport after a resize?

You should also try your code without SDL_FULLSCREEN and/or with a smaller window. I usually start with a 512×512 or 640×480 window until I get the viewport and some basic controls right.